There are various types of compound semiconductor devices. Among them, a high electron mobility transistor (HEMT) has low noise and is capable of operating at high speed. Exploiting these advantages, the HEMT is used in a base station for a mobile phone and the like.
In the HEMT, a two-dimensional electron gas induced in a carrier transit layer serves as carriers. Various materials can be used as the material of the carrier transit layer. When a nitride semiconductor such as GaN is used as the material of the carrier transit layer, the withstanding voltage of the HEMT can be increased due to a high band gap of the nitride semiconductor.
In order to induce the two-dimensional electron gas in a GaN layer which is the carrier transit layer, an AlGaN layer which has a lattice constant different from the GaN layer may be formed on the GaN layer. In this case, the lattice constant difference between the layers causes distortion in the AlGaN layer. Such a distortion induce piezoelectric polarization or spontaneous polarization in the AlGaN layer, which in turn induces the two-dimensional electron gas in the GaN layer which is the carrier transit layer.
Moreover, a barrier is formed at an interface between the GaN layer and the AlGaN layer because of different materials of the two layers, and the two-dimensional electron gas described above accumulates near the barrier.
The AlGaN layer is referred to also as a barrier layer, which induces the carrier in the carrier transit layer and which causes a barrier between the AlGaN layer and the carrier transit layer in this manner.
Note that the techniques related to the present application are disclosed in Japanese National Publication of International Patent Application No. 2007-538402 and Japanese Laid-open Patent Publication No. 2004-165387.
By the way, compound semiconductor layers which can be formed as the barrier layer include an InAlGaN layer in addition to the AlGaN layer described above. The InAlGaN layer has spontaneous polarization higher than that of the AlGaN layer and can thus induce a high-density two-dimensional electron gas in the carrier transit layer.
However, the InAlGaN layer has a wider band gap than the AlGaN layer. Accordingly, it is difficult to form ohmic contacts with the source electrode and the drain electrode. Hence, it is difficult to take out a drain current from these electrodes and this leads to a decrease in the drain current.